Lateral cross-sections of first guiding grooves provided in the inner wall surface of an outer cup are formed in a circular arc shape for one point contact with balls, and lateral cross-sections of second guiding grooves provided in the outer wall surface of an inner ring are formed in an elliptic arc shape for two-point contact with the balls. The diameter of a ball and an offset amount between a first and a second groove is set to satisfy the expression of 0.12.ltoreq.V.ltoreq.0.14 with V being the ratio of the two values.